As is well known, a circuit breaker is an automatically operated electro-mechanical device designed to protect a conductor from damage caused by a power overload or a short circuit. Circuit breakers may also be utilized to protect loads. A circuit breaker may be tripped by an overload or short circuit causing an interruption of power to the load. A circuit breaker can be reset (either manually or automatically) to resume power flow to the loads. One type of circuit breaker that provides instantaneous short circuit protection to motors and/or motor control centers (“MCC”) is called a motor circuit protector (MCP). A typical MCP includes a temperature-triggered overload relay, a circuit breaker, and a contactor. An MCP circuit breaker must meet National Electric Code (“NEC”) requirements when installed as part of a UL-listed MCC to provide instantaneous overload protection.
Mechanical circuit breakers energize an electromagnetic device such as a solenoid to trip a breaker instantaneously due to large surges in current such as by a short circuit. The solenoid is tripped when current exceeds a certain threshold. MCPs must protect against fault currents while avoiding tripping on in-rush motor currents or locked-rotor currents, but these current levels vary by motor. Existing MCPs have a relatively limited operating range, so they are suitable for protecting motor circuits within the MCP's operating range. For motor circuits outside of a particular MCP's operating range, a different MCP must be designed for the operating parameters of those motor circuits.
Fault currents are sensed by one or more current transformers that inductively couple a primary current into a secondary current according to a transfer function that defines a linear and saturation operating region of the current transformer. The transfer function of a current transformer shifts with temperature such that a higher secondary current output is produced for the same primary current input as temperature increases. The higher secondary current output has the effect of causing the MCP to trip sooner. To compensate for variances in environmental temperature, trip curves should be adjusted upwards or downwards. What is needed is a temperature compensation algorithm that automatically adjusts trip curve settings to compensate for changes in temperature.
Aspects of the various embodiments disclosed herein are directed to fulfilling these and other needs.